An antenna of a mobile communication device is typically is subdivided into an external antenna exposedly mounted to the outside thereof and an internal antenna which is mounted to the inside thereof in terms of its installation position. An external antenna such as a helical antenna or a whip antenna is protruded to the outside of the device, and hence has a high risk of damage. In addition, the external antenna has high standing wave ratio so that the radiation characteristic of transmission power is deteriorated to thereby increase the amount of the electric power consumed due to power output control. Also, the external antenna is not suitable for a trend toward miniaturization of the device since it is protrudingly mounted to the outside of the device. Thus, currently, the external antenna is being replaced with an internal antenna except for a communication scheme employing a low frequency band.
A conventional internal antenna basically has an inverted-F type or an inverted-L type structure in which a conductive radiator is disposed on a separate dielectric support element. Such an internal antenna can be made relatively small as compared to the external antenna, but a miniaturization of the communication device requires a much smaller antenna since the antenna still occupies a large space inside the device. In addition, as the function of the device becomes diverse and various communication services are introduced, the necessity increases that a single device must transmit/receive signals of various frequency bands. Therefore, it is required that the antenna also exhibits a multi-band characteristic. However, the conventional internal antenna is not proper for implementation of the multi-band characteristic under the limitation of a space for forming an antenna radiator.
A miniature spiral antenna using a spiral-shaped conductive radiator is disclosed in the International Publication No. WO 00/03453 of Ying et al., and U.S. Pat. No. 5,929,825 granted to Niu et al. However, Ying and Niu have implemented miniaturization of the antenna, but not a miniature internal antenna with the multi-band characteristic.